1. Field of the Invention
The present invention relates to thermoplastic polyolefin compositions which are flame retardant and which resist discoloration. More particularly, the invention relates to polystyrene and polypropylene thermoplastic polymers incorporating mixtures of hexabromocyclododecane and one or more bis-(polybromophenoxy)ethane which achieve a V-2 rating according to the UL-94 test and which resist discoloration at processing temperatures.
2. Description of the Prior Art
It is known in the art that polyolefins such as polystyrene, polypropylene homopolymer, and the like can be flame retarded with bromine-containing compounds. Among the flame retardant agents used in such compositions are hexabromocyclododecane, tetrabromobisphenol A, tetrabromobisphenol A bis-(dibromopropyl ether), bis-(tribromophenoxyethyl)tetrabromobisphenol A ether, octabromodiphenyl ether, decabromodiphenyl ether, bis-(tribromophenoxy)ethane, bis-(pentabromophenoxy)ethane, bis-(dibromopropyl carbonate) of tetrabromobisphenol A, and the like. For the most part, these flame retardant agents are used in sufficient quantity so that a rating of V-O is attained in the UL-94 test.
It is also well known that, for flame retarding polyolefins, aliphatic or alicyclic bromine-containing compounds are more effective than aromatic compunds (see J. Hoseczy, Soc. Plat. Eng., Tech. Pap., 20, 379-81). Certain applications, for example portable electronic devices, require a V-2 rating. For an effective V-2 formulation, the stability of the bromine compound and the extent of dripping during burning must be carefully balanced, taking into account the complex interaction of polymer substrate and bromine-containing compound (see C. Jarvis, Org. Coat. Plast. Chem., 43, 181-8). It is well known that polyolefin compositions which contain low concentrations of aliphatic or alicyclic bromine compounds, particularly hexabromocyclododecane, can achieve a V-2 rating (see J. Green, in Flame Retardants, Proc. Int. Symp., 1974, 68-77).
Although it has thus been proposed to incorporate various bromine-containing compounds, especially aliphatic bromine-containing compounds such as hexabromocyclododecane, in polyolefins that must meet V-2 standards, the compositions so obtained have a tendency to degrade on heating or during processing. For example, when such compositions are injection molded, the degradation normally results in severe discoloration of the composition.
In order to avoid polymer degradation problems, stabilizers have commonly been used to retard deterioration of the polymer arising from decomposition of the bromine-containing compounds or from interaction between the polymer and the bromine-containing compounds. Prior art disclosing the use of such stabilizers in polyolefins includes U.S. Pat. No. 4,089,912; Japanese Pat. Nos. 61-106,641; 61-57,636; 60-139,734; 56,145,939; and German Pat. No. 3,402,539 and a variety of others.
However, the use of such stabilizers in accordance with the prior art disturbs the drip/burn balance needed for V-2 formulations and increases the cost of V-2 rated polyolefin compositions. It is therefore desirable to have a V-2 flame retardant polyolefin composition which is resistant to discoloration yet which does not require incorporation of additives other than bromine-containing flame retardant.
Accordingly, a primary object of the present invention is to provide a flame retardant additive mixture capable of providing UL-94 V-2 thermoplastic polyolefin compositions.
A further object is to provide a flame retardant additive mixture of the character described that is especially useful in flame retarding normally combustible polystyrene and polypropylene.
A still further object is to provide a flame retardant additive mixture of the character described comprising agents which, when incorporated individually, do not permit a V-2 rating to be achieved.
Yet a further object is to provide a flame retardant additive mixture of the character described which allows the successful flame retarding of thermoplastic polyolefin compositions which resist discoloration without the use of additional stabilizers or other additives.